Gaging Tighter Tolerances Is A Different Animal

If you are planning on manufacturing to tolerances of 0. 0001 inch or less and have not done so before, your introduction to high order gaging might be an eye opener.

Columns
From:
10/1/2006
Modern Machine Shop,
George
Schuetz

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This ID/OD gage has all the elements necessary for millionth measurements—when used in a temperature controlled room—including a large granite base, set up with gage blocks, and millionth readout.

If you are planning on manufacturing to tolerances of 0.0001 inch or less and have not done so before, your introduction to high order gaging might be an eye opener. Gaging at this level is a very different animal, especially when considering how to measure the new parts you will be producing.

When gaging parts with the usual tolerances (0.0005 inch or broader), the quality of the gage is usually the main consideration. However, when you become interested in tolerances of say 30 microinches, there is much more than the gage to consider. You must place just as much importance on creating a special environment in which it will operate.

Even before thinking about the gage or the environment, however, a decision must be made about what should be measured: the size of the part or the relationship of the part to another part. The answer can make a big difference in the cost and difficulty of attaining the desired results. At these tighter tolerances, it is much less difficult and often more effective to check part relationships—clearance for example—rather than part dimensions.

If the decision is to check size, new methods of production measurement will be needed. Using the old 10× rule of thumb, for a 30 microinch tolerance, the repeatability of the gage needs to be in the 3 microinch range. To be sure both the gage and the master have such accuracy, we would need a master that is accurate to 0.3 microinches. Since no one would guarantee a master or even a gage block to have this accuracy, the best you can do is use the best standard available, which is apt to be a set of carefully cleaned and properly wrung Lab Master or Grade 0.5 (AAA) gage blocks with a ±1 microinch tolerance.

Now you need to focus on the environment. Within limits, temperature itself is not as important as its consistency and the degree to which it is shared by the gage, the master, the part and the surrounding atmosphere. For example, if you are checking the diameter of a 0.5-inch bore, a temperature difference of only one degree between the part and the master is likely to introduce an error of about 3 microinches. In addition, if the room temperature shifts a few degrees, or the operator’s body heat is allowed to reach the gage, it is easy to accumulate a few more millionths of error.

The parts, the master and even the gage should be part of one large heat sink so all can be brought to the same temperature and held there. The larger the mass of the gage and the heat sink, the less likely it will change during sudden air temperature fluctuations. Even with the heat sink in place, it may take hours for the parts to normalize. Gaging must take place in a constant-temperature room and the parts must be measured without being touched—the use of insulated gloves or tweezers is a must.

Assuming these recommendations are followed, it will still be necessary to check the gage frequently and compensate accordingly. No one can predict how the opening of a door and the resultant draft that occurs will affect the measurement. It might even be necessary to try to isolate the operator from the gaging process by placing a thermal shield between the operator and the gage.

The problem of dirt can be just as troublesome. Dirt particles or film, which are virtually invisible, can easily cause serious repetition errors. It will likely be necessary to chemically clean the entire gaging area periodically. Part cleaning will vary somewhat depending on the shape, material typeand size; however, because methods and cleaning fluids will have to be studied, residue can’t be ignored at these tolerances. Residue might also build up on gage contacts after repeated measurements.

Taking these steps into account—with thorough planning, careful personnel selection and training, along with significant investment in proper equipment—will help achieve measuring to these tight tolerances. Anything less can ruin the economics of the job and cost more in the long run.

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